Tens of thousands of teachers and support staff in Chicago are set to go on strike Monday after their union and school officials failed to reach a contract agreement, the union president said.

"Negotiations have been intense but productive, but we have failed to reach an agreement that would prevent a labor strike," Chicago Teachers Union President Karen Lewis told reporters late Sunday night.

Minutes earlier, the president of Chicago's school board said officials offered the city's teachers a contract including pay increases and other measures they'd requested.

"We've been as responsive as we know how," David Vitale told reporters just before 10 p.m. CT (11 p.m. ET) Sunday.

Vitale said the package offered by school officials include effectively guaranteed pay increases for four years, does not include merit pay and offers "some give on the evaluation system." He said that for nearly two hours, he'd tried to but been unable to talk with union president Karen Lewis.

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"The average teacher will get a 16% raise over that (4-year) period" at a time when the city's fiscal situation is on edge, the school board president said of the offered deal.

The looming strike in the nation's third largest school system -- the first in 25 years -- will affect nearly 700 schools and about 400,000 students, including some from neighborhoods struggling with crime and gang problems. The union itself has about 30,000 members.

For them, that would mean the school year would abruptly stop soon after it started: Some students in the district began class on August 13, and more -- on a different schedule -- started on September 4.

"In the morning, no (union) members will be inside our schools," Lewis said, adding that teachers will march on picket lines and talk to community members.